1. Field of the Invention
This invention relates generally to a thermal transfer recording system, and particularly to a thermal head adapted for inclusion in a thermal transfer recording system such as a word processor output device, a personal computer output terminal, or the like.
2. Description of the Prior Art
Recently, there has been proposed an edge-face type thermal head which enables high-speed printing to be affected on printing paper with a rough surface, without causing any trouble in the transportation of the thermal head (as described in Japanese Laid-Open Patent Publication No. 63-153165).
This type of thermal head is shown in FIG. 1, which comprises a flat substrate I of alumina or the like having a slanting surface 4 formed between a main surface 2 and an end surface 3 thereof, and also comprises a glaze layer 5 of an electrical insulating material formed on the main surface 2, the end surface 3 and the slanting surface 4. Further, an undercoat film 6 of SiO.sub.2 or the like is formed on the glaze layer 5, and a plurality of heat resistor layers 7 are formed on the portion of the undercoat film 6 which is located right above the slanting surface 4. Electrode films 8 and 9 are formed on the other portions of the undercoat film 6, extending from opposite ends of each of the heat resistor layers 7 along the main surface 2 and the end surface 3, respectively. The thermal head further includes a protective film 10 of SiO.sub.2 formed on the heat resistor layers 7 and part of electrodes 8 and 9 for wear resisting and anti-oxidation purposes.
One way to achieve higher-speed printing is to reduce the thickness of the protective film 10 shown in FIG. 1. However, since the protective film 10 is provided for the protection of the surface of the thermal head, the thickness of the protective film 10 cannot be reduced to a great degree.
Another conceivable way is to use a protective film having higher thermal conductivity. However, if the protective film 10 consists solely of such a protective film of higher thermal conductivity, as is the case with a conventional thermal head, the inherent function of the protective film 10 will be deteriorated, i.e., the temperature of the portion of the protective film 10 located right above the heat resistor layers 7 cannot reach a satisfactorily high level. This is apparent from the results of the thermal analysis simulation shown in FIG. 2, which shows that the temperature of the protective film located right above the heat resistor layers is decreased as the thermal conductivity of the protective film becomes higher. The reason is considered to be as follows: In the case where the thermal conductivity of the protective film 10 is high, the amount of heat transmitted from the heat resistor layers 7 to a heating area of the protective film 10 (located right above the heat resistor layers 7) is smaller than the amount of heat transmitted from the heating area of the protective film 10 to a non-heating area of the protective film 10 (located above the electrodes 8 and 9). Thus, heat is more readily conducted to the nonheating area than to the heating area, thereby decreasing the temperature of the heating area.
When a protective film of lower thermal conductivity is used, the temperature of the heating area of the protective film 10 does not become so high, as compared with the above case. Accordingly, the amount of heat transmitted from the heating area to the non-heating area of the protective film becomes small. Thus, the temperature of the heating area of the protective film 10 becomes eventually higher. In this case, however, it is difficult to raise the temperature of only the heating area of the protective film 10. This will prevent the thermal head from appropriately generating heat in accordance with print signals to be supplied form a signal generating means of the thermal transfer recording system, resulting in poor print quality.
Further, the use of a protective film having lower thermal conductivity will result in a relative increase in the flow of heat toward the undercoat film 6 and glaze layer 5 located right under the heat resistor layer 7. This causes poor thermal efficiency.
Another problem in the prior art is that, in order to decrease the size of the slanting surface 4 to allow the tip of the thermal head to further protrude, the thickness of the glaze layer 5 should be reduced. Accordingly, the heat insulating properties of the glaze layer 5 deteriorate, which increases the amount of heat to be transmitted into the glaze layer 5, resulting in increased power consumption.
A thermal head of a flat-face type which operates with good thermal efficiency for high speed printing is disclosed in Japanese Laid-Open Patent Publication No. 63-197664. This thermal head includes a glaze projection formed on a substrate of alumina or the like and protruding from the substrate to be readily brought into contact with printing paper. On the glaze projection are formed a heating element and electrodes connected to the heating element to supply current thereto. Protective films of different materials are disposed further thereon in such a manner that the thermal conductivity of the protective film on the heating element is set to be higher than that of the protective film on the other area. In such a thermal head, the heat generated by the heating element is readily conducted upward to the protective film just above the heating element, while the flow of heat to the protective film on the other area is suppressed. The purpose of this arrangement is to improve heat efficiency and to attain high speed printing.
This type of thermal head, however, cannot be used for printing on paper with a rough surface for the following reason: If this flat-face type thermal head is to be used for printing on a rough sheet of printing paper, the glaze projection of the thermal head must be of a double-layered structure to further protrude from the substrate. For that purpose, the lower glaze layer of the double-layered glaze projection should be made larger in thickness, which makes the whole glaze projection larger in thickness to a great degree. Thus, a considerable amount of heat generated by the heating element is accumulated in the glaze layers, resulting in increased power consumption. It is also impossible to attain high speed printing. With such a thermal head, it is difficult to carry out bidirectional printing because the substrate of the head interferes with such operation.
As described above, a thermal head of this type comprises protective films of different levels of thermal conductivity so as to improve thermal efficiency for the reduction of power consumption, but it cannot be used for printing on a rough sheet of printing paper or for bi-directional printing to attain higher speed printing.